1. Field of the Invention
This invention relates generally to apparatus for monitoring selected conditions of liquid, such as engine coolant in a vehicular radiator, and more particularly to a sensor which can be used to provide remote indication of the degree of corrosivity of the coolant, the presence or absence of the coolant at a selected level in the radiator or both conditions as desired.
2. Description of the Prior Art
Vehicular cooling systems are composed of several components which include the radiator, circulating pump, passages in the engine block and associated tubing. Since the system is composed of metallic parts, there is a need to prevent, or at least mitigate, corrosion in order to prolong the useful life of the system. To this end it has become common practice to add chemical substances to the coolant liquid which serve to inhibit corrosion of the metal surfaces which come in contact with the liquid. Such substances are known as inhibitors and generally form a film on the metal surfaces thereby protecting them from corrosion. Thus, commercially available permanent antifreeze includes an inhibitor which is effective in preventing corrosion, however, over the course of time the corrosion inhibiting characteristic of the liquid can become less effective due to various factors.
At the present time the use of aluminum for cylinder heads and cooling systems in automobiles is becoming more common. Aluminum cylinder heads run hotter than iron cylinder heads. This combined with aluminum's higher susceptibility to corrosion leads to the potential for a phenomena known as "hot transport corrosion." This is a process whereby aluminum corrosion products are transported from the hottest areas, typically in the head, to the coolest, that is, the radiator. These contaminents interfere with good heat transfer and degrade the heat rejection capability of the system. Aluminum heads are also less tolerant to overheat conditions than iron and run the risk of warping at elevated temperatures which can result from a low coolant condition. Also, the aluminum used in radiators and heater cores is more susceptible to corrosion than the traditional copper-brass system which makes proper maintenance more critical to prevent failure due to perforation.
In U.S. Pat. No. 4,147,596, assigned to the assignee of the present invention, a system is disclosed and claimed in which a potential measuring circuit employing at least two electrochemical electrodes composed of dissimilar metals is located so as to be immersed and in contact with coolant liquid. The electrodes are mounted in a tubular housing which is provided with a threaded portion so that it can be screwed into a threaded bore of a wall confining the coolant liquid. When the inhibiting characteristic is effective, a first range of electrical potential exists between the electrodes; however, when the inhibiting characteristic becomes ineffective for any reason, a second range of electrical potential exists therebetween. Upon reaching a threshold level, as the potential moves into the second range, indicating means are actuated to provide a suitable indication of the condition.
In U.S. Pat. No. 4,253,064, also assigned to the assignee of the present invention, another condition of the coolant is sensed utilizing the same electrodes which are part of a coolant inhibition characteristic condition sensing system. In that system the presence or absence of liquid is determined by making use of the electrical resistance between the two electrodes without causing any appreciable current through the liquid which current would adversely effect the ability to sense the coolant inhibition characteristic condition. The resistance between the electrodes serves as part of a feedback network in a feedback oscillator. The feedback oscillator incorporates a dc measuring amplifier for measuring the electrode potential as an indication of the corrosion inhibition characteristic of liquid in which the electrodes are immersed. The same amplifier provides the amplification necessary for oscillation when the resistance between the electrodes increases to a threshold level indicating an absence of liquid. The feedback network includes a capacitor which not only determines the frequency of oscillation, along with appropriate resistance, but also blocks any possible DC current which might otherwise flow through the electrodes. The sensing of liquid presence or absence is accomplished between a first electrode and ground while the second electrode may be directly grounded, coupled to ground through a large capacitor or resistor or separated from the first electrode by an electrically insulative barrier and thus coupled to ground through the liquid clinging to the insulator separating the second electrode from ground.
However the use of the feedback network in both the coolant inhibition characteristic sensing function and the level sensing function tends to limit the flexibility of the system. For example, adjusting the threshold sensing parameters of one function sometimes may affect the other function. The use of the feedback network in both functions also subjects the system to some degree of noise sensitivity which could cause nuisance tripping under certain conditions. Another limitation of the prior art involves the fact that the electrochemical sensing of corrosivity requires the sensing of negative potentials. In dealing with this the prior art provided a negative power supply which rendered the system more complex and costlier than desirable. Another limitation was the output format of the system (DC output for corrosivity and AC output for low level). This made sensor output difficult to decode.
It is an object of this invention to sense electro-chemical potentials of electrodes in a liquid, including potentials below ground, as an indication of corrosivity of the liquid as well as sensing the presence or absence of the liquid using a common set of electrodes. Another object is the provision of a system which can be used to sense either the function of corrosion inhibition effectiveness of a liquid or the function of the presence or absence of such liquid or both functions if desired. Yet another object is the provision of a sensor used for either or both functions which is readily mounted on a vehicular cooling system which is inexpensive yet reliable and long lasting. Another object is the provision of a sensing system for sensing the functions of liquid corrosivity and liquid level for which the threshold values for the function are readily and independently calibratable and one which has selected hysteresis which is also calibratable. It is another object of the invention to provide an engine coolant condition sensor assembly which is inexpensive to manufacture and is easily installed in a liquid reservoir in a tamper proof manner. Another object is the provision of such a sensor assembly which houses not only the sensing elements but also electronics used to process or condition the electrical signal generated by the sensing elements and transmit it to a remote location, such as the instrument panel of an automobile, to give visual or audio indication of the condition of the coolant.
Other objects, advantages and details of the apparatus provided by the invention appear in the following detailed description of the preferred embodiment of the invention.